Published and Distributed by theTexas Water Development BoardP.O. Box 13231Austin, TX 78711-3231

Executive Summary

In April of 2007, the Texas Water Development Board entered into agreement with theU.S. Army Corps of Engineers, Fort Worth District, for the purpose of performing a volumetricand sedimentation survey of Lewisville Lake. The U.S. Army Corps of Engineers, Fort WorthDistrict, contributed 50% of the funding for this survey through their Planning Assistance toStates Program, while Dallas Water Utilities contributed the remaining 50%. This survey wasperformed using a multi-frequency (200 kHz, 50 kHz, and 24 kHz) sub-bottom profiling depthsounder. In addition, sediment core samples were collected in selected locations and were usedin interpreting the multi-frequency depth sounder signal returns to derive sedimentaccumulation estimates.Lewisville Dam and Lewisville Lake are located on the Elm Fork Trinity River inDenton County, Texas. Bathymetric data collection for Lewisville Lake occurred between July18thand September 21stof 2007, while the water surface elevation ranged between 532.77 feetand 524.55 feet above mean sea level (NGVD29). Additional data was collected on January30th, April 15th, May 13th-14th, and May 21st, of 2008, while the water surface elevationaveraged 521.0 feet, 523.6 feet, 522.0 feet, 522.0 feet, and 521.9 feet above mean sea level(NGVD29), respectively. The conservation pool elevation of Lewisville Lake is 522.0 feetabove mean sea level (NGVD29).The results of the TWDB 2007 Volumetric Survey indicate Lewisville Lake has atotal reservoir capacity of 598,902 acre-feet and encompasses 27,175 acres at conservationpool elevation. Previously published capacity estimates for Lewisville Lake are 648,400 acre-feet, 640,986 acre-feet, and 571,926 acre-feet, based on surveys conducted in 1960, 1965, and1989, respectively.1Due to differences in the methodologies used in calculating capacitiesfrom this and previous Lewisville Lake surveys, comparison of these values is notrecommended. TWDB recommends that a similar methodology be used to resurvey LewisvilleLake in approximately 10 years or after a major flood event.The results of the TWDB 2007 Sedimentation Survey indicate Lewisville Lake hasaccumulated 28,603 acre-feet of sediment since impoundment in 1954. Based on thismeasured sediment volume and assuming a constant sediment accumulation rate, LewisvilleLake loses approximately 540 acre-feet of capacity per year. The thickest sediment deposits arelocated between Lewisville Dam and the breached Lake Dallas dam within the main body ofthe lake. The maximum sediment thickness observed in Lewisville Lake was 5.9 feet.

Lewisville Dam and Lewisville Lake are located on the Elm Fork Trinity River, atributary of the Trinity River, in the Trinity River Basin 22 miles northwest of Dallas inDenton County, Texas (Figure 1). Lewisville Lake is owned and operated by the U.S.Army Corps of Engineers, Fort Worth District. Construction on Lewisville Dam began onNovember 28, 1948, with deliberate impoundment beginning on November 1, 1954. Theproject was completed in August of 1955.2

During the 1920’s the City of Dallas built a dam on the Elm Fork of the TrinityRiver upstream from the current Lewisville Dam. This dam was completed in 1927,creating Lake Dallas. However, significant silt accumulation in Lake Dallas prompted theU.S. Army Corps of Engineers to construct Lewisville Dam.3In 1957 the original dam wasbreached and Lake Dallas became part of Lewisville Lake.3Lewisville Lake serves mainlyas a water supply source for the Cities of Dallas and Denton and surrounding communities,and also provides flood control and recreation.4Additional pertinent data about LewisvilleDam and Lewisville Lake can be found in Table 1.

The water rights for Lewisville Lake have been appropriated to the City of Dallasthrough Certificate of Adjudication No. 08-2456 and its amendments, and to the City ofDenton through Certificate of Adjudication No. 08-2348. A brief summary of thecertificates and each amendment follows. The complete certificates are on file in theRecords Division of the Texas Commission on Environmental Quality.

3

•Certificate of Adjudication No. 08-2456 Issued: July 22, 1983 and re-issued asAmendment to Certificate of Adjudication No. 08-2456A on November 10, 1983

Authorizes the City of Dallas to store 549,976 acre-feet of water in Lewisville Lake, up toelevation 522 feet above mean sea level. The City of Dallas is authorized to divert and use fromLewisville Lake a maximum of 403,700 acre-feet of water per annum for municipal water supplyfor the City of Dallas; a maximum of 134,976 acre-feet of water per annum for municipal anddomestic purposes; a maximum of 800 acre-feet per annum from the reservoir and 9,500 acre-feetper annum from the Elm Fork Trinity River for industrial purposes; and a maximum of 1,000 acre-feet per annum for domestic purposes. Authorizes a time priority of January 25, 1924 to store214,000 acre-feet of water, divert 293,700 acre-feet of water per annum for industrial use and 1,000acre-feet for domestic use. Authorizes a time priority of October 5, 1948 to store 201,000 acre-feetof water and divert 110,000 acre-feet of water per annum for municipal purposes. Authorizes a timepriority of November 24, 1975 to store 134,976 acre-feet of water and divert 134,976 acre-feet perannum for municipal and domestic use.

In addition to the uses currently authorized under Certificate No. 08-2456, as amended, theCity of Dallas is also authorized to use a maximum of 451,030 acre-feet of water per annum of the549,976 acre-feet currently authorized for use by the city, for diversion from Lewisville Lake fornon-consumptive hydroelectric purposes on a non-priority basis.

Authorizes the City of Dallas to change the purpose of use of the 9,500 acre-feet of waterper annum currently authorized in Certificate 08-2456, as amended, from industrial use to industrialand municipal use, and to relocate the point of diversion on the Elm Fork Trinity River to a pointapproximately 1 mile upstream from the existing point of diversion.

Authorizes the City of Dallas, in addition to existing authorizations, to store in LewisvilleLake, within the currently authorized capacity of 549,976 acre-feet, a maximum of 97,200 acre-feetof treated wastewater effluent return flows delivered by pipeline from Dallas’ Central WastewaterTreatment Plant and Southside Wastewater Treatment Plant. In addition to the existing diversionauthorization, the City of Dallas is also authorized to divert a maximum of 97,200 acre-feet of waterper year of the documented amount of return flows, less carriage and evaporative losses, dischargedby the City of Lewisville, the Town of Flower Mound, the Dallas Central Wastewater Treatment

4Plant, and the Dallas Southside Wastewater Treatment Plant for the purposes authorized byCertificate No. 08-2456, as amended. The time priority for the use of the treated effluent inDecember 5, 2001. The City of Dallas will continue to discharge 114,000 acre-feet of water peryear of treated effluent from Dallas’ Central and Southside Wastewater Treatment Plants and leavethat amount in the Trinity River Basin for instream flows.

In lieu of the previous “purpose of use” authorizations, this amendment authorizes the Cityof Dallas to divert and use a maximum 549,976 acre-feet of water per year (of which 540,476 acre-feet is from Lewisville Lake and 9,500 acre-feet is from the Elm Fork Trinity River) for multiplepurposes (municipal, domestic, agricultural (irrigation), industrial, and recreation) and 451,030 acre-feet of water per year for non-consumptive hydroelectric purposes (on a non-priority basis).Authorizes a time priority of January 25, 1924 for the diversion and use of 305,000 acre-feet peryear, a time priority of October 5, 1948 for the diversion and use of 110,000 acre-feet of water peryear, and a time priority of November 24, 1975 for the diversion and use of 134,976 acre-feet ofwater per year.

•

Certificate of Adjudication No. 08-2348 Issued: July 22, 1983

Authorizes the City of Denton to store 68,424 acre-feet of water in Lewisville Lake up toelevation 522 feet above mean sea level. The City of Denton is authorized to divert and use amaximum of 58,424 acre-feet of water per annum for municipal and domestic purposes within thearea served or to be served by the city’s distribution system, and to other communities in theLewisville Dam Watershed. Authorizes a time priority of November 24, 1948 to store 21,000 acre-feet of water and divert and use 11,000 acre-feet of water for municipal and domestic purposes.Authorizes a time priority of November 24, 1975 for the storage and use of 47,424 acre-feet ofwater.

5

Volumetric and Sedimentation Survey of Lewisville Lake

The Texas Water Development Board’s (TWDB) Hydrographic Survey Programwas authorized by the state legislature in 1991. The Texas Water Code authorizes TWDBto perform surveys to determine reservoir storage capacity, sedimentation levels, rates ofsedimentation, and projected water supply availability.In April of 2007, TWDB entered into agreement with the U.S. Army Corps ofEngineers, Fort Worth District, for the purpose of performing a volumetric andsedimentation survey of Lewisville Lake. The U.S. Army Corps of Engineers, Fort WorthDistrict, contributed 50% of the funding for this survey through their Planning Assistance toStates Program, while Dallas Water Utilities contributed the remaining 50%. These surveyswere performed simultaneously using a single-beam multi-frequency (200 kHz, 50 kHz,and 24 kHz) sub-bottom profiling depth sounder. The 200 kHz return measures the currentbathymetric surface, while the combination of the three frequencies, along with coresamples for correlating the pre-impoundment surface with the signal return, is analyzed forevidence of sediment accumulation throughout the reservoir.

Datum

The vertical datum used during this survey is that used by the United StatesGeological Survey (USGS) for the reservoir elevation gage USGS 08052800 Lewisville Lknr Lewisville, TX.6The datum for this gage is reported as National Geodetic VerticalDatum 1929 (NGVD29) or mean sea level, thus elevations reported here are in feet abovemean sea level. Volume and area calculations in this report are referenced to water levelsprovided by the USGS gage. The horizontal datum used for this report is North AmericanDatum 1983 (NAD83) State Plane Texas North Central Zone.

TWDB Bathymetric Data Collection

Bathymetric data collection for Lewisville Lake began on July 18thand continuedthrough September 21stof 2007. During the survey the water surface elevation ranged from532.77 feet to 524.55 feet above mean sea level (NGVD29). Additional data was collectedon January 30th, April 15th, and May 13th, May 14th, and May 21st, of 2008, while the water

6surface elevation averaged 521.0 feet, 523.6 feet, 522.0 feet, 522.0 feet, and 521.9 feetabove mean sea level, respectively. For data collection, TWDB used a Specialty Devices,Inc., multi-frequency sub-bottom profiling depth sounder integrated with DifferentialGlobal Positioning System (DGPS) equipment. Data collection occurred while navigatingalong pre-planned range lines oriented perpendicular to the assumed location of the originalriver channels and spaced approximately 500 feet apart. The depth sounder was calibrateddaily using a velocity profiler to measure the speed of sound in the water column and aweighted tape or stadia rod for depth reading verification. During the survey, teammembers collected approximately 410,500 data points over cross-sections totaling nearly569 miles in length. Figure 2 shows where data points were collected during the TWDB2007 survey.

Figure 2 - Data points collected during TWDB 2007 Survey

7Data Processing

Model Boundaries

The reservoir boundary was digitized from aerial photographs, also known asdigital ortho quarter-quadrangle images (DOQQs)7,8, using Environmental SystemsResearch Institute’s (ESRI)

ArcGIS 9.1 software. The quadrangles that cover LewisvilleLake are Little Elm, Denton East, Lewisville East, and Lewisville West. Each quarter-quadrangle image was photographed on September 10, 2004, August 3, 2004, or August30, 2004, during which time the water surface elevation at Lewisville Lake measured522.15 feet, 523.42 feet, and 522.39 feet above mean sea level, respectively. Thesephotographs have a 1-meter resolution; therefore, the physical lake boundaries may bewithin ± 1 meter of the location derived from the manual delineation. As the majority ofthe lake is represented by photos taken on September 10, 2004 while the water surfaceelevation measured 522.15 feet, the boundary was digitized at the land water interfacevisible in the photos and labeled 522.0 feet, or conservation pool elevation.More recent aerial photographs of Lewisville Lake were taken on August 2, 2006,August 9, 2006, and August 19, 2006, while the water surface elevation measured 512.24feet, 512.03 feet, and 511.66 feet, respectively. From these, a 512.0 foot contour, verifiedfor accuracy against the data collected during the survey, was digitized to supplement theTWDB survey data in locations where the survey data alone was insufficient to properlyrepresent the reservoir bathymetry.

Triangulated Irregular Network (TIN) Model

Upon completion of data collection, the raw data files collected by TWDB wereedited using HydroEdit and DepthPic to remove any data anomalies. HydroEdit is used toautomate the editing of the 200 kHz frequency and determine the current bathymetricsurface. DepthPic is used to display, interpret, and edit the multi-frequency data intandem to correct any edits HydroEdit has flagged and to manually interpret the pre-impoundment surface. The water surface elevations at the times of each sounding areused to convert sounding depths to corresponding bathymetric elevations. For processingoutside of DepthPic, the sounding coordinates (X,Y,Z) are exported as a MASS pointsfile. TWDB also created a MASS points file of interpolated data located in-between

8surveyed cross sections. This point file is described in the section entitled “Self-SimilarInterpolation.” To represent reservoir bathymetry in shallow regions, additional pointswere added using the “Line Extrapolation” technique.9These MASS points files alongwith the boundary files are used in creating a Triangulated Irregular Network (TIN) modelwith the 3D Analyst Extension of ArcGIS. The 3D Analyst algorithms use Delaunay’scriteria for triangulation to place a triangle between three non-uniformly spaced points,including the boundary vertices.10

Using Arc/Info software, volumes and areas are calculated from the TIN model forthe entire reservoir at one-tenth of a foot intervals, from elevation 452.9 feet to elevation522.0 feet. The Elevation-Capacity Table and Elevation-Area Table, updated for 2007,are presented in Appendix A and B, respectively. The Area-Capacity Curves arepresented in Appendix C.The TIN model was interpolated and averaged using a cell size of 2 feet by 2 feetand converted to a raster. The raster was used to produce Figure 3, an Elevation ReliefMap representing the topography of the reservoir bottom, Figure 4, a map showing shadeddepth ranges for Lewisville Lake, and Figure 5, a 5-foot contour map (attached).

Self-Similar Interpolation

A limitation of the Delaunay method for triangulation when creating TIN modelsresults in artificially-curved contour lines extending into the reservoir where the reservoirwalls are steep and the reservoir is relatively narrow. These curved contours are likely apoor representation of the true reservoir bathymetry in these areas. Also, if the surveyedcross sections are not perpendicular to the centerline of the submerged river channel (thelocation of which is often unknown until after the survey), then the TIN model is notlikely to well-represent the true channel bathymetry.To ameliorate these problems, a Self-Similar Interpolation routine (developed byTWDB) was used to interpolate the bathymetry in between many survey lines. The Self-Similar Interpolation technique effectively increases the density of points input into theTIN model, and directs the TIN interpolation to better represent the reservoirtopography.13In the case of Lewisville Lake, the application of Self-Similar Interpolationhelped represent the lake morphology near the banks and improved the representation ofthe submerged river channel (Figure 6). In areas where obvious geomorphic features

11indicate a high-probability of cross-section shape changes (e.g. incoming tributaries,significant widening/narrowing of channel, etc.), the assumptions used in applying theSelf-Similar Interpolation technique are not likely to be valid; therefore, Self-SimilarIinterpolation was not used in areas of Lewisville Lake where a high probability of changebetween cross-sections exists.9

Figure 6 illustrates typical results of the application of theSelf-Similar Interpolation routine in Lewisville Lake, and the bathymetry shown in Figure6C was used in computing reservoir capacity and area tables (Appendix A, B).

Figure 6 - Application of the Self-Similar Interpolation technique to Lewisville Lakesounding data – A) bathymetric contours without interpolated points, B) Sounding points(black) and interpolated points (red) with reservoir boundary shown at elevation 522.0(black), C) bathymetric contours with the interpolated points. Note: In 6A the submergedriver channel indicated by the surveyed cross sections is not represented for the areas in-between the cross sections. This is an artifact of the TIN generation routine. Inclusion ofthe interpolated points (6C) corrects this and smoothes the bathymetric contours.

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Survey Results

Volumetric Survey

The results of the TWDB 2007 Volumetric Survey indicate Lewisville Lakehas a total reservoir capacity of 598,902 acre-feet and encompasses 27,175 acres atconservation pool elevation (522.0 feet above mean sea level, NGVD29). In 1960 theU.S. Army Corps of Engineers estimated the capacity of Lewisville Lake (at conservationpool elevation, 522.0 feet above mean sea level) at 648,400 acre-feet.1,2The U.S. ArmyCorps of Engineers resurveyed Lewisville Lake in 1965 and estimated the capacity to be640,986 acre-feet. In 1989, Turner Collie & Braden Inc. calculated a reservoir capacity of571,926 acre-feet.1Due to differences in the methodologies used in calculating areas andcapacities from this and previous Lewisville Lake surveys, comparison of these values isnot recommended. TWDB considers the 2007 survey to be a significant improvementover previous methods and recommends that a similar methodology be used to resurveyLewisville Lake in approximately 10 years or after a major flood event.

Sedimentation Survey

The 200 kHz, 50 kHz, and 24 kHz frequency data were used to interpret sedimentdistribution and accumulation throughout Lewisville Lake. Figure 7 shows the thicknessof sediment throughout the reservoir. To assist in the interpretation of post-impoundmentsediment accumulation, ancillary data was collected in the form of five core samples.Sediment cores were collected on May 14, 2008 using a Specialty Devices, Inc.VibraCore system.The results of the TWDB 2007 Sedimentation Survey indicate Lewisville Lakehas accumulated 28,603 acre-feet of sediment since impoundment in 1954. Based onthis measured sediment volume and assuming a constant sediment accumulation rate,Lewisville Lake loses approximately 540 acre-feet of capacity per year. The majority ofthe sediment accumulation has occurred between the current Lewisville Lake dam and thebreached Lake Dallas dam within the main body of the lake. This suggests that some ofthe Lake Dallas sediments may have been re-deposited further downstream in LewisvilleLake after the 1957 Lake Dallas dam breach. The maximum sediment thickness observed

13in Lewisville Lake was 5.9 feet. A complete description of the sediment measurementmethodology and sample results is presented in Appendix D.

TWDB Contact Information

More information about the Hydrographic Survey Program can be found at:

The results of the TWDB 2007 Sedimentation Survey indicate Lewisville Lakehas accumulated 28,603 acre-feet of sediment since impoundment in 1954. Based on thismeasured sediment volume and assuming a constant rate of sediment accumulation overthe 53 years since impoundment, Lewisville Lake loses approximately 540 acre-feet ofcapacity per year. This sediment accumulation rate is likely to overestimate the actualrate of sediment influx to Lewisville Lake, as it is impossible to determine the amount ofsediment that accumulated in Lake Dallas before creation of Lewisville Lake. Thethickest sediment deposits are located between the current Lewisville Lake dam and thebreached Lake Dallas dam within the main body of the lake. The maximum sedimentthickness observed in Lewisville Lake was 5.9 feet.

Introduction

This appendix includes the results of the sediment investigation using multi-frequency depth sounder and sediment core data collected by the Texas WaterDevelopment Board (TWDB). Through careful analysis and interpretation of the multi-frequency signal returns, it is possible to discern the pre-impoundment bathymetricsurface, as well as the current surface and sediment thickness. Such interpretations areaided and validated through comparisons with sediment core samples which provideindependent measurements of sediment thickness. The remainder of this appendixpresents a discussion of the results from and methodology used in the core sampling andmulti-frequency data collection efforts, followed by a composite analysis of sedimentmeasured in Lewisville Lake.

D1

Data Collection & Processing Methodology

TWDB conducted the main Lewisville Lake bathymetric survey between July18th, 2007 and September 21st, 2007 and collected additional data on January 30th, April15th, May 13th, May 14th, and May 21stof 2008. For all data collection efforts, TWDBused a Specialty Devices, Inc. (SDI), multi-frequency (200 kHz, 50 kHz, and 24 kHz)sub-bottom profiling depth sounder integrated with Differential Global PositioningSystem (DGPS) equipment. Data collection occurred while navigating along pre-plannedrange lines oriented perpendicular to the assumed location of the original river channelsand spaced approximately 500 feet apart. For all data collection efforts, the depth sounderwas calibrated daily using a velocity profiler to measure the speed of sound in the watercolumn and a weighted tape or stadia rod for depth reading verification. During the 2007survey, team members collected approximately 410,500 data points over cross-sectionstotaling nearly 569 miles in length. Figure D1 shows where data points were collectedduring the TWDB 2007 Lewisville Lake survey.Core samples collected by TWDB were collected at locations where soundingdata had been previously collected (Figure E1). All cores were collected with a custom-coring boat and SDI VibraCore system. Cores were analyzed by TWDB, and both thesediment thickness and the distance the core penetrated the pre-impoundment boundarywere recorded. The coordinates and a description of each core sample are provided inTable D1. Figure D2 shows the cross-section of sediment core L3. At this location,TWDB collected 20 inches of sediment, with the upper sediment layers (Figure E2)having high water content, consisting of clay material and lacking in vegetation. The pre-impoundment boundary was evident from this core at a distance of 10 inches above thecore base; above this location, the moisture content in the sediment greatly increases(Figure E2).

Figure D2 – Sediment Core L3 from Lewisville Lake, showing the pre-impoundmentboundary 10 inches above the base of the core (left). The pre-impoundment boundary ismarked by the change in sediment moisture content below and above the area 10 inchesup from the core base.

All sounding data is processed using the DepthPic software, within which boththe pre-impoundment and current bathymetric surfaces are identified and digitizedmanually. These surfaces are first identified along cross-sections for which core sampleshave been collected – thereby allowing the user to identify color bands in the DepthPicdisplay that correspond to the sediment layer(s) observed in the core samples. Thisprocess is illustrated in Figure D3 where core sample L3 is shown with its correspondingsounding data. Core sample L3 contained 20 inches of sediment above the pre-impoundment boundary, as indicated by the yellow and green boxes, respectively,representing the core sample in Figure D3. The pre-impoundment surface is usuallyidentified within the core sample by one of the following methods: (1) a visualexamination of the core for in-place terrestrial materials, such as leaf litter, tree bark,twigs, intact roots, etc., concentrations of which tend to occur on or just below the pre-impoundment surface, (2) changes in texture from well sorted, relatively fine-grainedsediment to poorly sorted mixtures of coarse and fine-grained materials, and (3)variations in the physical properties of the sediment, particularly sediment water contentand penetration resistance with depth.

D4

Figure E3 – DepthPic and core sample use in identifying the pre-impoundmentbathymetry.

Within DepthPic, the current surface is automatically determined based on thesignal returns from the 200 kHz transducer. The pre-impoundment surface must bedetermined visually based on the pixel color display and any available core sample data.Based on core sample L3, it is clear that sediment layer is indicated by the pink and redpixels. The pre-impoundment bathymetric surface for this cross-section is therefore

D5identified as the base of the bright-colored pink pixels in the DepthPic display, and thecurrent bathymetric surface is located at the top of the band of red and pink pixels.(Figure E3).In analyzing data from cross-sections where core samples were not collected, theassumption is made that sediment layers may be identified in a similar manner as whencore sample data is available. To improve the validity of this assumption, core samplesare collected at regularly spaced intervals within the lake, or at locations whereinterpretation of the DepthPic display would be difficult without site-specific core data.For this reason, all sounding data is collected and reviewed before core sites are selectedand cores are collected.After manually digitizing the pre-impoundment surface from all cross-sections,both the pre-impoundment and current bathymetric surfaces are exported as X-,Y-,Z-coordinates from DepthPic into text files suitable for use in ArcGIS. Within ArcGIS, thesounding points are then processed into TIN models following standard GIS techniques1.The accumulated sediment volume for Lewisville Lake was calculated from a sedimentthickness TIN model created in ArcGIS. Sediment thicknesses were computed as thedifference in elevations between the current and pre-impoundment bathymetric surfacesas determined with the DepthPic software. Sediment thicknesses were interpolated forlocations between surveyed cross-sections using the TWDB self-similar interpolationtechnique2. For the purposes of the TIN model creation, TWDB assumed 0-feet sedimentthicknesses at the model boundaries (defined as the 522.0 foot NGVD29 elevationcontour).Results

The results of the TWDB 2007 Sedimentation Survey indicate Lewisville Lake hasaccumulated 28,603 acre-feet of sediment since impoundment in 1954. The thickestsediment deposits are located between Lewisville Dam and the breached Lake DallasDam within the main body of the lake. The maximum sediment thickness observed inLewisville Lake is 5.9 feet. Figure D4 depicts the sediment thickness in Lewisville Lake.Based on the measured sediment volume in Lewisville Lake and assuming aconstant rate of sediment accumulation over the 53 years since impoundment, Lewisville

D6Lake loses approximately 540 acre-feet of capacity per year. This sediment accumulationrate is likely an overestimation of the actual rate of sediment influx to the lake, as it isdifficult to determine the amount of sediment that accumulated in Lake Dallas beforecreation of Lewisville Lake. The relatively thick sediment deposits in Lewisville Lakebetween the Lake Dallas dam and the Lewisville Dam suggests that Lake Dallassediments may have been re-deposited further downstream in Lewisville Lake after the1957 Lake Dallas dam breach. Accounting for the lifespan of Lake Dallas (from 1927 to1957), an alternative sediment accumulation rate estimate for Lewisville Lake (over theperiod 1927-2007) is 353 acre-feet per year. To improve the sediment accumulation rateestimates, TWDB recommends Lewisville Lake be re-surveyed using similar methods inapproximately 10 years or after a major flood event.